1. Field of the Invention
The invention relates to a mobile communication system which transfers a variety of items of multimedia data including voice in a packet data form such as an Internet Protocol (IP) packet, and a gateway, a concentrator, and a handover control method for use in the system of this kind. More specifically, the present invention relates to an improvement of handover processing accompanied by movement of a mobile terminal device.
2. Description of the Related Art
A mobile communication system including a personal handy-phone system (PHS) or a so-called cellular phone system has been expected to spread in future. Fusing with an IP network such as the Internet has been planed as well as extension of a communication bandwidth, and in recent years, transmission of the IP packet by means of a method of encapsulating and tunneling has been taken in account.
As widely known, the mobile communication system includes a plurality of base stations, and each base station individually forms a radio zone (refer to as an area or a cell). In an area, a radio terminal device (refer to as a mobile terminal) is wirelessly connected to the base station which develops the area. A mobile terminal moving to another area switches a communication partner from a station being in connection to a base station at a moving destination. This processing is called handover.
The communication between the mobile terminal and the base station is deteriorated as the mobile terminal comes close to an area boundary. In the existing mobile communication system, a method in which the mobile terminal detects this deterioration in communication, and the mobile terminal requires reconnection (calling) to the next base station is adopted (e.g., refer to Jpn. Pat. Appln. KOKAI Publication No. 2005-27119).
Meanwhile, in the existing technique, a packet loss accompanied by temporary deterioration of a communication environment before and after handover may not be avoided. While voice communication may overcome a slight packet loss, there is the danger of a loss of important information in transmission of an image or text data. Especially, if the mobile terminal moves at high speed, the situation gets worse because the handover is performed frequently and many packets are lost.
To prevent the packet loss, a method for confirming whether or not a mobile terminal may normally receive the image or the text data for each packet at every time is a possible approach. However, this method has to wait a response to confirm OK/NG of receptions, deteriorates a communication rate, and uselessly consumes the communication bandwidth and network resources. Retransmission control through a high order layer (TCP/IP) poses the same situation.
A technique aiming to solve such a failure is disclosed in Jpn. Pat. Appln. KOKAI. Publication No. 2003-153327. This document discloses a system which predicts a packet at a moment of switching of base stations and reports the packet number of the predicted packet to an exchanger. However, since such a system is burdened with prediction computation and a band for message transfer between the base station and the exchanger is occupied, a further effective method in stead of the system has been searched.